The question arises: how do scientists determine that the Earth is 4.54 billion years old, that the last Ice Age (when glacial coverage was at its maximum) occurred 10,000 years ago, or that fossils dating back 90 million years belong to species B? How do scientists ascertain the age of rocks, fossils, or geological events that happened billions of years ago?
To start, there are three main types of rocks in the Earth’s crust: igneous rocks (formed from magma that rises from beneath the surface and cools), sedimentary rocks (such as sand transported by rivers from upstream, which settles under favorable conditions to form layers that compress into rock), and metamorphic rocks (which are altered from sedimentary or igneous rocks under conditions of heat and pressure).
Sedimentary conglomerate formed through the deposition, binding, and compression of pebbles.
Regarding the age of geological objects (rocks, fossils, etc.), there are two types: relative age (A is younger than B, B is older than C) and absolute age (determining exactly how many millions of years ago a rock formed). For example, in the image above, the pebble was actually broken, transported, and eroded from another type of rock upstream. We cannot determine the absolute age of the pebble and conclude that it represents the formation age of the conglomerate downstream; that would be a mistake. In this case, paleontologists will examine the organisms that were fossilized and buried along with that pebble, then determine the age of those fossils (based on characteristics of genus, species, etc.), from which they can estimate the relative formation age of the conglomerate.
In the example below, rock C is known to cut into rock B (which has an established age of 1.1 million years) but is covered by rock A (which has an established age of 1 million years). Thus, the relative formation age of rock C is between 1.1 and 1 million years.
The method for determining the absolute age of rocks is based on isotopic systems. For instance, the Uranium-Lead (U/Pb) isotopic system shows that igneous rocks, after cooling, contain Uranium (the parent isotope), which over time decays into Lead (the daughter isotope). Using specialized equipment, scientists can count the remaining parent atoms and the number of daughter atoms at the present time and calculate the formation time using the radioactive decay equation:
Where λ is the decay constant, D* is the number of daughter isotopes produced, and N is the number of remaining parent isotopes.
There are many isotopic systems that can be applied to determine the absolute age of rocks. As summarized in the table below, the C14 method can date organic materials such as coal, petrified wood, fossils, and bones.
The methods for absolute dating can be quite expensive and still have certain inaccuracies. It is hoped that advancements in technology will address these two issues in the future.
SHRIMP (Sensitive High-Resolution Ion Microprobe) dating system.
In Vietnam, the oldest rock found so far is approximately 2.9 billion years old, located in the northwest region (Lào Cai, Yên Bái).
Speaking of Earth science, there are many intriguing questions. Over the timeline, will humanity face extinction like the second age of dinosaurs? What life form will follow? … and many more questions that require clearer answers from humanity.
